The subsurface layer reference to calculate preformed alkalinity and air–sea CO2 disequilibrium in the Atlantic Ocean

The subsurface ocean layer (100–200 m deep) is suggested as a reference to parameterise preformed alkalinity (AT°) and air–sea CO2 disequilibrium (∆Cdis) in the Atlantic. Results suggest that this domain retains the memory of water mass formation (WMF) conditions over annual periods and avoids the large, short-term variability of the uppermost layers. Its thermohaline variability also encompasses and represents all water masses that outcrop in the Atlantic. Subsurface data also avoids the scarcity of late wintertime surface observations and benefit from the larger availability of year-round measurements, thus enhancing their representativeness and application coverage. This last feature is most relevant in high Atlantic latitudes, where WMF typically occurs and the widespread ice sheets often preclude surface pCO2 sampling during wintertime. The obtained AT° and ΔCdis parameterisations achieve uncertainties of ± 4.6 and ± 5.6 μmol kg− 1, respectively, improving significantly the estimates in previous works, particularly in the high latitudes. The AT° parameterisation is well correlated with observations and is coherent with the latitudinal subsurface distribution of silicate, particularly in the northern subpolar region, where previous studies showed discrepancies. The ΔCdis estimates in the upper layers are coherent with air–sea ∆pCO2 data from Takahashi's climatology, thus tackling known important shortcomings and biases of anthropogenic CO2 estimates in Atlantic waters.